Not applicable.
1. Technical Field
The present invention relates to valves, and in particular, to multistage electrohydraulic servo valves.
2. Description of the Related Art
Electrohydraulic servo valves are well-known, particularly for use in pilot stages of directional control valves. One such application is for an actuator operating the compressor bleed valve of an aircraft turbofan engine. Electrohydraulic servo valves can have a first stage with an electrical or electromagnetic force motor controlling flow of a hydraulic fluid driving a valve member, such as a spool valve, of a second stage, which in turn can control flow of hydraulic fluid to an actuator driving the load. The force motor can operate to position a movable member, such as a flapper, in response to an input drive signal to drive the second stage valve member. Electrical or mechanical feedback can be provided to return the force motor to the original or null position after the valve member has been moved to its desired position, thereby stopping its movement.
U.S. Pat. No. 4,456,031 discloses one example of an electrohydraulic servo valve. In this case, the valve first stage has a torque motor driving an armature to pivot a flapper member toward and away from two nozzles through which hydraulic fluid can be directed at either of opposing ends of a spool so as to move the spool and thereby control flow to an actuator. Redundant mechanical spring and electrical transducer feedback systems are employed to prevent shut-down of the valve in the event of failure of malfunction of one of the feedback systems. See also U.S. Pat. No. 5,249,603. However, these and other existing systems are disadvantageous in that they do not exhibit both high response and low null leakage, competing attributes that are highly advantageous in hydraulic systems.
Accordingly, an improved multi-stage valve is desired.
The present invention provide a two stage electrohydraulic servo valve in which the first stage has an inherently balanced variable flow valve member that is auto-nulled by a feedback force from associated movement of the valve member in the second stage. The stroke of the hydraulically driven second stage valve member is proportional to a drive signal input to the first stage.
Specifically, the invention provides an electrohydraulic servo valve having first and second stage units. The first stage servo valve unit has a drive assembly adapted to move a forked clevis member from a null position to alternatively open and close first and second nozzle orifices. When the first nozzle orifice is open flow is permitted between a pressure port and a control port and when the second nozzle orifice is open flow is permitted between the control port and a return port. The second stage valve unit has a sliding valve member as well as an inlet port in communication with the first stage control port, a flow port and a pressure port. Flow from the second stage pressure port to the flow port is controlled by the sliding valve member. The clevis member and the sliding valve member are linked by a feedback spring such that movement of the sliding valve member imparts a feedback force to the feedback spring to return the clevis member to the null position.
In one preferred form, the sliding valve member is a spool cooperating with a half-area piston held stationary by fluid pressure and disposed within a fixed guide sleeve. The spool member moves under the force of flow from the first stage control pressure to close off flow from the second stage pressure or return ports.
In other forms, the valve can have a two-way second stage in which the flow is controlled from the second stage pressure port (with no return port) to a single metering flow port. Or, the valve can have a three-way second stage having a pressure port, a return port and a single input/output flow port. Or, the valve can have a four-way second stage in having pressure, return and two input/output flow ports, in which case flow is discharged from one flow port and taken in through the other flow port.
In another preferred form, wherein the drive assembly is a permanent magnet motor having a wire coil and a movable actuator member connected to the clevis member and disposed along a main axis. The first stage valve unit further includes a flexure pivot allowing the clevis member to pivot with respect to the main axis to control flow through the nozzle orifices. The flexure pivot has a movable part and a non-moving part in a plane spaced from the movable part and joined thereto by a flexible spoke.
In still other preferred forms, the feedback spring has a ball end that is pivotally engaged with the socket or groove in the spool to alleviate binding. The first stage valve unit can include a separate valve body defining the nozzle orifices, which are preferably two pairs of slots through opposite flat sides of the nozzle body. The nozzle body can be partitioned and have a bore through the partition through which the feedback spring extends. In this case, one nozzle orifice is on each side of the partition. Further, the forked end of the clevis member has two prongs, one disposed on opposite sides of the nozzle body. Preferably, each prong has tapered lateral leading edges.
The present invention thus provides an improved electrohydraulic servo valve that is both highly responsive and exhibits low null leakage. These and other benefits are derived in large part to the use of the clevis valve member in the first stage. The clevis arrangement is inherently pressure balanced such that it is highly insensitive to the affects of pressure loading and transient flow forces as well as to pump pressure ripple or noise common in hydraulic or fuel systems, which works to maximize the net drive force during operation. The valve is also highly efficient, empirically exhibiting very high first stage pressure recovery (approximately 97%) and very low hysteresis. The valve is also highly reliable and suitable for use in highly particle contaminated environments, such as jet fuel applications because of a high first stage pressure gain working to clear the spool in the event of sticking. In addition, the valve arrangement is closed centered in that the clevis prongs close the nozzle orifices at the null position such that the valve provides very low null leakage with variable transient flow.
These and still other advantages of the invention will be apparent from the detailed description and drawings. What follows is a preferred embodiment of the present invention. To assess the full scope of the invention the claims should be looked to as the preferred embodiment is not intended as the only embodiment within the scope of the invention.